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32                                         PICRIC ACID

oxalate, etc.    A third portion is mixed with 4 vols. of alcohol.    In no case
should a precipitate form.

9. Organic Matter.—When strongly heated in a. dish, the acid blackens
if organic substances are present. 5 c.c. of the acid are boiled for 5
minutes with 5 c.c. of dilute silphuric acid and 5 drops of 0-1% perman-
ganate : in presence of organic matter, the liquid becomes decolorised
(the decoloration may, however, depend on lower acids of phosphorus,

arsenious acid, etc.).

*

Meta-phosphoric or glacial -phosphoric acid (HFC)., = 80) is sold in glassy
masses or rods. It may contain the. same impurities as ordinary phosphoric
acid but is contaminated more particularly with sodium metaphosphate,
often in considerable quantities. This impurity is detected by dissolving
the acid in concentrated hydrochloric acid (I) ™ 1-19) : the presence of
the sodium salt leads to the formation of sodium chloride, which remains
undissolved.

H3CnN807 - 239

Lemon-yellow, crystalline scales or powder, ni.pt. 122-123", soluble
in 25 parts of cold water, readily soluble in boiling water, alcohol, ether
or benzene. The commercial acid may contain, as impurities, resinous
matters, oxalic acid, sulphates, chlorides and mono- and di-nitrophenols,
and may be adulterated with considerable quantities of alum, magnesium
sulphate, sodium sulphate and sodium chloride.

1.  Resinous and Various Insoluble Substances.    4 grams of the
acid arc boiled with loo c.c. of water until completely dissolved, any insoluble
residue being collected on a tared filter and weighed.

2.  Various Mineral Salts.-—4 grams of the acid arc treated with 100
c.c. of ether, any insoluble residue, being collected, weighed and analysed,
tests being made especially for magnesium sulphate, alum, sodium sulphate
and sodium chloride.

3.  Oxalic Acid.—-4 grams dissolved in hot water (about; 250 c.c.) are
neutralised with ammonia and tested with calcium chloride.

4.  Sulphuric Acid.—4 grams, dissolved in 250 c.c. of water, are tested
with barium chloride,

5.  Hydrochloric Acid .--4 grams, dissolved in 250 c.c. of water and
acidified with nitric acid, arc tested with silver pitrate solution.

6.  Mono-   and   di-nitrophenols.- -Into   two   bottles   with   ground
stoppers are poured equal volumes of i% bromine water, into one of them
a i% solution of the picric acid and then into each excess of potassium
iodide.   The iodine liberated in each case is titrated with N/io-thiosulphatc
and from the bromine combined the content of these two impurities cal-
culated.

The pure acid should not leave more than 0-1% of residue insoluble in water
or 0-2% insoluble in ether. Only traces of oxalic acid or hydrochloric acid
should be found, and the sulphuric acid should not exceed 0-05%,n alcohol. Its most frequent impurities art; sulphates,